A molecular pump is a type of vacuum pump, which takes advantage of a high rotating wheel of a rotor for delivering momentum to gas molecules so as to provide them with desired direction speed, thus the gas shall be inhibited and driven towards the exhaust, and then be pumped by a forestage pump. A magnetic levitation molecular pump is a molecular pump that takes a magnetic bearing (also known as active magnetic levitation bearing) as a bearing for the rotor of the molecular pump, and the rotor is suspended in the air stably through the magnetic bearing, so that no mechanical contact exists between the rotor and the stator during the rotation of the rotor with high speed. Thus, the magnetic levitation molecular pump has several advantages such as no mechanical attrition, low energy consumption, allowed rotating with high speed, low noise, long life-time, no requirement of lubrication, etc. Currently, the magnetic levitation molecular pumps are widely applied to field of vacuum production devices for obtaining high vacuum and high cleanliness vacuum environment.
The inner structure of the magnetic levitation molecular pump is shown in FIG. 1. The rotor of magnetic levitation molecular pump comprises a rotor shaft 7 and a wheel 1 in fixed connection with the rotor shaft 7. The wheel 1 is fixed on the upper portion of the rotor shaft 7; the rotor shaft 7 is successively covered with a first radial magnetic bearing 6, a motor 8, and a second radial magnetic bearing 9 etc. in a separated manner. The above listed assemblies together constitute the rotor shaft system of the magnetic levitation molecular pump.
After completing assemble of the magnetic levitation molecular pump, imbalance mass may exist in the rotor because of several problems such as differences between processing accuracy of every part of the rotor. Imbalance mass means mass in a certain distance from the barycenter of the rotor, and the product of this mass and centripetal acceleration equals to the rotor's imbalance centrifugal force. When the imbalance mass is much larger than 10 mg, due to this imbalance mass, an eccentric moment may be generated between the barycenter of the rotor and its axis. Therefore, during the rotating ascend of the rotor, the centrifugal force caused by the imbalance mass of the rotor may cause transverse mechanical vibration of the rotor (usually radial vibration), and further to impact normal work of the system. In addition, normal working speed of the rotor of the magnetic levitation molecular pump is remaining within a high speed range that is beyond the rigid critical speed of the rotor, and the imbalance mass may also block the rotor from rotating with an increasing speed till normal working speed, i.e. the magnetic levitation molecular pump cannot work normally. Here, the rigid critical speed of the rotor indicates a rotating speed that is in correspondence to a speed when the rotating frequency of the rotor equals to the rigid resonance frequency of the rotor bearing system; while the high speed range which higher than rigid critical speed may be known as super rigid critical rotating speed range.
A method for inhibiting imbalance vibration generated during the acceleration and deceleration of the rotating member rotating with high speed such as the rotor of the magnetic vibration molecular pump, and the method is called “method of controlling imbalance vibration”. Chinese Periodical Literature “a method of controlling imbalance vibration of magnetic levitation bearing system” (Dekui, ZHANG, Wei, JIANG, Hongbin, ZHAO, Journal of Tsinghua University (science and technology) 2000, Volume 40, No. 10) discloses two methods of controlling imbalance vibration. One is force free control, the principle of which is to generate a compensation signal having a same phase and a same amplitude with a displacement/vibration signal of the rotor, in order to counteract same frequency of vibration of the rotor. The other method is open loop feed forward control, the principle of which is extracting a same frequency component of a vibration signal of the rotor, and then generating a corresponding controlling signal by an extra feed forward control, which may be added into a controlling signal of a main controller.
Chinese patent literature CN101261496A discloses a high-precision control system for active vibration of magnetic levitation wheel, comprising a displacement sensor, a current sensor, a controller for magnetic bearing and an power amplifier for magnetic bearing, wherein, the controller for magnetic bearing comprises a stability controller, a eccentric estimation unit, a magnetic force compensation unit and a switch. Based on stability control, the patent also introduces the eccentric estimation unit and the magnetic force compensation unit as well as taking advantage of wheel imbalance vibration parameter so as to make compensation to imbalance values and negative displacement rigidity within allowed rotating speed of the wheel. Therefore, imbalance vibration within allowed rotating speed of the wheel is controlled, furthermore, the wheel can rotate around the inertia axis with high-precision during the entire acceleration and deceleration of the wheel. Additionally, Chinese patent literature CN 101046692A discloses an open loop high-precision control system for imbalance vibration of magnetic bearing reaction wheel, comprising a displacement sensor, an interface circuit for displacement signal, a detector for rotating speed, a controller for magnetic bearing, a power amplifier circuit for magnetic bearing and a detector for position of the wheel. The controller for magnetic bearing comprises an axial controller and a radial controller, wherein, the axial controller comprises a controller for stability and a controller for imbalance vibration that is adapted for providing compensation to displacement feedback of the controller for stability. Based on stability controlling, imbalance vibration controlling is incorporated. By using imbalance vibration parameter of the wheel detected during the high-speed rotation of the wheel, as well as using current position of the rotor of the wheel detected by the detector for position of the wheel, imbalance vibration is controlled in an open loop high-precision manner within the allowed rotating speed of the wheel, thus imbalance vibration controlling is achieved within the entire allowed rotating speed of the wheel, which ensures the wheel to rotate with a high-precision during its acceleration and deceleration.
The two aforesaid patent literatures disclose specific applications of “method for rotor imbalance vibration control”. However, due to limited ability of imbalance vibration control by using “method for rotor imbalance vibration control”, which means it is required the imbalance mass of the rotating member staying within a certain threshold. Therefore, the method for imbalance vibration control cannot completely solve the problem of the rotor vibration caused by rotor's imbalance mass. When there is a large imbalance mass in the rotor, the “method for rotor imbalance vibration control” cannot be used any more to suppress vibration of the rotor and to accelerate the rotor directly beyond the rigid critical rotating speed till its normal working rotating speed.
Thus, after assembling the magnetic levitation molecular pump, rotor dynamic balancing operation is required to be executed. Here “rotor dynamic balancing operation” means an operation for adjusting and eliminating imbalance mass through measuring weight and phase of the imbalance mass of the rotor, so that no centrifugal force shall be generated during the rotation of the rotor.
Generally, a rotor dynamic balancing device is used for executing rotor dynamic balancing operation, with steps of:
firstly, rotating the rotor at a low speed (slower than rigid critical rotating speed of the rotor), and operating rotor dynamic balancing through the rotor dynamic balancing device;
then, adding or removing weight to the rotor in a purpose of balance so as to eliminate its imbalance mass preliminarily;
then, repeating the above listed steps till the rotating speed of the rotor being accelerated beyond rigid critical rotating speed;
when the rotating speed of the rotor is accelerated into the range of super rigid critical rotating speed, operating rotor dynamic balancing again through the rotor dynamic balancing device at a high speed, and then adding or removing weight to the rotor thereafter. In order to eliminate imbalance mass accurately, the aforesaid steps are required to be repeated for several times.
When the rotor of magnetic levitation molecular pump is rotating at a speed in the range of super rigid critical rotating speed, we focus on the performance of the rotor at high rotating speed, thus the effect of operating rotor dynamic balancing when the rotor is rotating at low speed is not obvious. Only when the rotating speed of the rotor is much more faster than the rigid critical rotating speed (entering the range of super rigid critical rotating speed), the rotor may rotate around its barycenter approximately. At this moment, operating rotor dynamic balancing may get more accurate and better effects. However, due to the existed imbalance mass, the rotor cannot be accelerated directly into the range of super rigid critical rotating speed, furthermore cannot be operated with rotor dynamic balancing operation at a high speed. Therefore, it is required to operate the rotor with rotor dynamic balancing operation when the rotor is rotating at a low speed and then accelerate it till the range of super rigid critical rotating speed, and operate it with rotor dynamic balancing operation again at high speed. This method of dynamic balancing of the rotor is quite inconvenient and inefficient. Also, the rotor dynamic balancing device used for operating dynamic balancing is commercially available, and people purchase this device in addition for dynamic balancing operation to the rotor, which may increase the cost of their products.